The rapid determination of CO2 in expired air is one of the more important measurements in respiratory physiology. Of the CO2 transducers currently employed, the condenser microphone and thermal conductivity types have several disadvantages. The former is sensitive to mechanical and acoustical vibrations. The latter is sensitive to other gases and lacks response time. In most cases the over-all reliability leaves something to be desired. The CO2 transducer developed to alleviate these difficulties employs a Fabry-Perot type interference filter and a lead selenide photoconductive infrared detector. The resulting device has rapid response time, is stable mechanically, and is simple electronically.